Many enclosures that contain sensitive instrumentation must maintain very clean environments in order to operate properly. Examples include enclosures with sensitive optical surfaces or electronic connections that are sensitive to particles and/or gaseous contaminants that can cause contamination or corrosion. Other examples are data recording devices such as computer hard disk drives that are sensitive to particles, organic vapors, and corrosive vapors. Still other examples include electronic control boxes such as those used in automobiles that are sensitive to moisture buildup as well as contamination from fluids and vapors.
Pressure sensitive adhesive filter assemblies and adsorbent vent assemblies are well known. Diffusion tubes are also common and are incorporated into the filter and adsorbent assemblies in one of two ways. The first common technique involves incorporating the diffusion tube directly into a plastic housing. Such a device is described in U.S. Pat. No. 4,863,499 in which a tortuous path is created between the plastic body portion and plastic cover portion of the assembly. The creation of this tortuous path requires the use of molded plastic channels to be incorporated into the body portions when manufactured. The portions are then ultrasonically welded together, snap locked or adhesively bonded to form a one piece unit.
Similarly U.S. Pat. No. 5,030,260 requires a diffusion plate which becomes part of the lower housing. This diffusion plate is likewise provided with channels that cooperate with other parts of the assembly thus creating a chamber for the air from the outside to enter and then diffuse through before entering the chamber.
The limitations of these types of assemblies are numerous. Initial costs to create the molds for plastic injection and molded parts are expensive. In addition, because the chambers or tortuous path created in either of these devices are formed by the cooperation of the different component parts, the elements such as the channel built within the component part must be made precisely. Moreover, the size and spacing of these assemblies has become increasingly critical as miniaturization of control boxes and data storage devices has increased. For example, computer disk drives have decreased in size from 5.25 inches (133.4 mm) disks down to 1.3 inches (33 mm) disks. The space available for a diffusion tube filter/adsorbent assembly has significantly diminished thereby severely limiting the availability for use of the molded plastic type filter assemblies.
An alternate diffusion filter assembly involves creating a groove directly onto the external surface of the housing enclosure and then covering this groove with a planar surface. Again significant cost and feasibility limitations are present. The housing typically is constructed from aluminum, stainless steel, polyvinyl chloride, polycarbonate, or other moldable metal or polymer. The addition of a groove into any of these materials is very costly and labor intensive. Moreover, since the diffusion tubes effectiveness is dependent on the size of the groove or tube through which the air passes, it is important that the groove be small in diameter. The precision of this groove as well as the limited flexibility and versatility further limits the usefulness of this technique.
Finally, retrofitting an existing enclosure with a diffusion groove is both infeasible and cost prohibitive.
There is a need for an improved and miniaturized filter/adsorbent assembly having a diffusion tube that increases protection from harmful particulate and vapor contamination in an enclosure that can be easily applied or retrofitted to an enclosure that has an existing vent hole opening.